Thoth Amon is a powerful spellcaster, and is definitely hard for the unprepared. He can curse your items, destroy your armor, and even summon nasties. Thus, waltzing up and stabbing him may be a bad idea, especially without magic resistance.

Those who have a wand of cancellation in their arsenal can use it to prevent him from casting spells, as he has very low MR. Without spellcasting, he just deals 1d6+1d4 damage - easily manageable for a level 14 barbarian. Another great option is using a wand of sleep to disable him while you pound him into the ground. Or simply zap him with a wand of death.